In a rollover judging method for use in conventional rollover judging devices, a two-dimensional mapping method of mapping an angular velocity ω and a tilt angle θv is used, and a method of setting ON and OFF regions on mapping in advance, and of, when (ω, θv) is pointed in the ON region, triggering an occupant crash protection device, such as an air bag, is used. As a rollover judging device which employs another example of the two-dimensional mapping method, there is the following conventional example.
In the mapping method for use in this conventional rollover judging device, in order to start an occupant crash protection according to a rotation of a vehicle in its rightward or leftward direction, a judgment threshold is set up for each of the first quadrant and the third quadrant, and this judgment threshold is changed according to the acceleration Gy of the vehicle in its rightward or leftward direction. More specifically, the judgment threshold itself is set as a variable, and the angular velocity ω and the tilt angle θv also change according to the behavior of the vehicle (for example, refer to patent reference 1).
In addition to the above-mentioned reference, there have been provided following patent references 2 to 4 as conventional technologies relevant to rollover judgment.    [Patent reference 1] JP,2001-71844,A    [Patent reference 2] Japanese Patent No. 3715146    [Patent reference 3] JP,2001-71787,A    [Patent reference 4] JP,2001-74442,A
Conventional rollover judging devices are constructed as mentioned above, and, in these conventional rollover judging devices, the judgment threshold itself is set as a variable, and the angular velocity ω and the tilt angle θv also change according to the behavior of the vehicle. A problem is therefore that for each of the first quadrant and the third quadrant, a computation of the judgment threshold and a judging process are needed, and the process is complicated.
A further problem with the process using the two-dimensional mapping method in the above-mentioned conventional example (patent reference 1) is that it is difficult to show how much it gets close to the threshold in actual vehicle driving tests, i.e., to express the severity of the rollover phenomenon using a simple percentage.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a rollover judging device which has a simple structure without using a complicated process like the above-mentioned two-dimensional mapping method, which makes it possible to show the severity of the rollover phenomenon using a simple percentage, and which simplifies arithmetic processing by setting a judgment threshold not as a variable but as a fixed value, thereby improving its reliability.